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The first and the most important is mpulsivity, and a failure to evaluate implications. They feel the urge to get a hamster, so they do. They feel like taking a trip, so they book the next flight. A high need for novelty and stimulation is one of the strongest correlates with infidelity. Get an idea about some more signs in this cool article about red flags of a toxic relationship https://headway.media/7-red-flags-in-relationships/ I respect the most.